1. Field of the Invention
The present invention relates to control systems for electric arc furnaces.
2. Description of the Related Art
An electric arc furnace (EAF) is a furnace that heats material by way of an electric arc. Arc furnaces range in size from small units of approximately one ton capacity up to about 400 ton units used for secondary steelmaking. On a much smaller scale arc furnaces for use in research laboratories and by dentists have a capacity of tens of grams. Industrial electric arc furnace temperatures can typically be up to 1,800° C., and laboratory units can exceed 3,000° C. Arc furnaces directly expose material to an electric arc, and the current in the furnace electrodes pass through the material.
An EAF generally includes a refractory-lined vessel covered with a retractable roof, through which one or more graphite electrodes enter the furnace. The EAF is primarily split into three sections: the shell, which consists of the sidewalls and lower steel “bowl”; the hearth, which is the refractory layer that lines the lower bowl; and the roof, which may be refractory-lined and/or water-cooled, and can be shaped as a section of a sphere, or as a conical section. The roof also supports the refractory through which the graphite electrodes enter.
A typical alternating current EAF is powered by a three-phase electrical supply having three electrodes that enter through the roof. Electrodes are typically round in cross-section, and are arranged in segments with threaded couplings, so that as the electrodes wear, new segments can be added. The arc forms between the material in the EAF and the electrode, the material is heated both by current passing through the material and by the radiant energy from the arc. The electrodes are raised and lowered by a positioning system, which may use either electric winches or hydraulic cylinders. The regulating system maintains approximately constant current and power input during the melting of the material, even though scrap may move under the electrodes as it melts. The mast arms holding the electrodes can be coupled with busbars to carry the electrical current or the mast arms may be “hot arms”, where the whole arm carries the current. Hot arms may consist of copper-clad steel or aluminum. The electrodes move up and down for regulation of the arc, and are raised to allow removal of the furnace roof.
The EAF is often coupled to a tilting platform so that the liquid steel can be poured therefrom. A typical EAF could have a transformer rated about 60,000,000 volt-amperes (60 MVA), with a secondary voltage between 400 and 900 volts and a secondary current in excess of 44,000 amperes. Such a furnace would be expected to produce a quantity of 80 tons of liquid steel in approximately 50 minutes from charging the EAF with cold scrap to tapping the furnace. In comparison, basic oxygen furnaces can have a capacity of 150-300 tons per batch, or “heat”, and can produce a heat in 30-40 minutes.
The process to melt the steel includes the lowering of the electrodes onto the scrap, causing an arc to be struck and the electrodes are then set to “bore” into the layer of scrap at the top of the furnace. Typically lower voltages are selected for this first part of the operation to protect the roof and walls from excessive heat and damage from the arcs. Once the electrodes have reached the heavy melt at the base of the furnace and the arcs are shielded by the scrap, the voltage is increased and the electrodes raised slightly, lengthening the arcs and increasing power to the melt. This enables a molten pool to form more rapidly, reducing tap-to-tap times.
What is needed in the art is a controller to optimize the EAF performance, to efficiently produce molten steel.